Veritasium
July 21, 2026
Understanding Schrödinger's Cat Thought Experiment
TL;DR
Schrödinger's cat demonstrates quantum superposition by showing how a cat in a sealed box with a radioactive atom would theoretically be both alive and dead simultaneously until observed, illustrating the counterintuitive nature of quantum mechanics at macroscopic scales.
Key Takeaways
- 1Schrödinger's cat was designed as a thought experiment to illustrate how quantum superposition applies to large, observable objects, not just atoms
- 2The cat's fate is entangled with the quantum state of the radioactive atom—if the atom decays, poison is released and the cat dies; if not, the cat lives
- 3According to quantum mechanics, before measurement, the atom exists in a superposition of both decayed and not decayed states simultaneously
- 4This superposition extends to the cat through entanglement, meaning the cat is theoretically both alive and dead at the same time
- 5The wave function collapses only when the box is opened and observed, forcing the cat into either a dead or alive state
Notable Quotes
“Since the state of the cat and detector apparatus are directly tied to the state of the atom, we say they are entangled.”
“According to quantum mechanics the cat really is both alive and dead at the same time. Only when we open the box and make a measurement does the wave function collapse and the cat actually becomes either dead or alive.”
Chapters
1. The Thought Experiment Setup
A cat is placed in a sealed box with a radioactive atom, a radiation detector, and a mechanism that releases poisonous cyanide gas if radiation is detected. The cat's survival depends entirely on whether the atom decays.
2. Quantum Superposition and Entanglement
The radioactive atom exists in a superposition of both decayed and not decayed states. Since the detector and cat are directly tied to the atom's state, they become entangled with this superposition, creating a scenario where the cat is in a superposition of both alive and dead.
3. The Measurement Problem
The wave function describing the system remains in superposition until the box is opened and observed. Only then does the wave function collapse, forcing the cat to become definitively either alive or dead.
Key People & Entities
- Schrödinger
- Physicist who created this famous thought experiment to illustrate quantum mechanics paradoxes
Glossary
- Superposition
- A quantum state in which a particle or system exists in multiple states simultaneously until measured or observed
- Entanglement
- A quantum phenomenon where two or more systems become correlated such that the state of one directly influences the state of the other
- Wave Function
- A mathematical description of the quantum state of a system that contains all information about the system until it is measured
- Wave Function Collapse
- The process by which a quantum superposition resolves into a single definite state upon measurement or observation